Controlling factors and reservoir characteristics of Dolostone-evaporite co-occurring systems in the Eastern Sichuan Basin, China

The dolostone-evaporite co-occurring system (DECS) is widely found in Precambrian to Quaternary strata. In the eastern Sichuan Basin, DECS is mainly developed in the Cambrian Longwangmiao, Gaotai, and Xixiangchi Formations. In this paper, the controlling factors of sedimentary, sedimentary models, and main controlling factors of reservoir of the DECS of the eastern section of the Sichuan Basin were investigated, through core observations, thin section analysis, and field profiles measurement and sampling, combined with well-logging data, interpretations of geochemical analyses, and cathodoluminescence. The lithological associations of the evaporite and dolostone in the DECS include interbedded dolostone and evaporites, thick-bedded to mass evaporites above the dolomite, dolostone intercalated with evaporite, and evaporite intercalated with dolostones. The reservoir of DECS is mainly a dolomite reservoir in dolomite interbedded evaporite. The lithology mainly consists of oolitic dolomite and siliceous fine crystalline dolomite. The reservoir porosity includes dissolution porosity, porosity, and fractures. The analysis of the geochemical characteristic indicated that the formation of DECS is affected by the closed water body and high salinity (Z > 120), the hot and arid paleoclimate (T = 21.1 degree celsius), intense evaporation, and the continuous decline of the ancient sea level. According to the paleoenvironmental characteristic, the models of DECS include four dolomite models, evaporite pump (sabkha) dolomite diagenetic model, reflux permeable dolomite diagenesis model, diagenetic model of buried dolomite, and tectonic-age hydrothermal dolomite genesis model, and two evaporite models: Sabkha diagenetic model and underwater concentration diagenetic model. Based on previous studies, the influence of different factors on the deposition of the shoals in the DECS was evaluated. The main controlling factors for the formation of high-quality reservoir units include thermochemical sulfate reduction (TSR) and structure-controlled hydrothermal dolomitization. Sulfides and deep fluids in DECS play a constructive role in reservoirs.